Absorption spectrophotometry of a measurement object gas using, for example, FTIR (Fourier Transformation Infrared Spectrometer) uses a multiple reflection type cell of a white type having multiple reflecting surfaces in order to lengthen an optical path length of light passing the measurement object gas while shortening a cell length of a cell chamber into which the measurement object gas is introduced (refer to patent document 1, 2).
For this multiple reflection type cell, a reflecting member having the reflecting surface is adjusted by a position adjusting mechanism such as a set screw or a drawing thread in order to obtain a desired optical path length. Ordinarily, the position adjusting mechanism is housed in a cell chamber in order to simplify a cell structure. With this structure, it is necessary to take an inner volume of the cell chamber large in order to house the position adjusting mechanism.
In case of measuring a time change of a component of the measurement object gas by the use of the multiple reflection type cell, it is required the responsiveness should be improved by shortening time to replace the introduced measurement object gas as much as possible and decreasing the inner volume of the cell chamber.
In order to attain this object, the multiple reflection type cell described in the patent document 1 has a structure wherein a block body is arranged in a dead space of the cell chamber so as to decrease the inner volume of the cell chamber.
However, this multiple reflection type cell reduces the dead space of a lateral side (a peripheral part locating at a position orthogonal to the optical path) of the optical path of the multiple reflection, and does not reduce a dead space resulting from the position adjusting mechanism of the reflecting member. In addition, since this structure requires a block body provided separately, not only the structure becomes complicated but also a dead space where the measurement sample gas resides is formed in a gap between the block body and the inside surface of the cell chamber. Furthermore, the structure wherein a dead space resulting from the position adjusting mechanism is formed or the block body is provided becomes an obstacle to downside the multiple reflecting type cell.
A multiple reflecting type cell described in the patent document 2 has a gas container into which the sample is introduced and three reflecting members arranged in both sides of the gas container, and the sample gas and the reflecting members are separated by a window of the gas container. With this structure, it is possible to exclude the dead space resulting from the position adjusting mechanism.
However, with this structure, it is necessary to provide an additional structure (concretely, a brewster window) in order to reduce reflection of the light when the light passes a window of the gas container. In addition, since the sample gas and the reflecting member are separately arranged, not only the optical path length between the sample gas and the reflecting member becomes wasteful but also whole of the cell increases in size to gain a desired optical path length.